Study on influence of coal surface functional groups on methane and carbon dioxide adsorption properties

The influence of coal surface functional groups on the adsorption properties of CH4 and CO2 has been confirmed, but the specific promotion and hindrance of different functional groups on the adsorption behavior of the two gases as well as the influencing factors have not been determined. By using density functional theory, the adsorption energy data of different functionalized structures on CH4 and CO2 were calculated, and the action rules of surface functional groups on the adsorption performance of CH4 and CO2 were revealed. The results show that whether the structure modified by functional groups or non-functionalized C-layer structure, the adsorption energy of CO2 is higher than that of CH4, which indicates that the interaction between CO2 and coal macromolecular structure is greater than that of CH4. In the adsorption energy simulation of CH4/CO2 competitive adsorption, the adsorption equilibrium distance of CH4 is larger than that in single CH4 molecule adsorption simulation. The adsorption equilibrium distance of CO2 is smaller than that of single CH4 molecule in the simulation of CH4/CO2 competitive adsorption energy, and CO2 shows competitive adsorption advantage. The order of adsorption energy of CH4 in different oxygen-containing functionalized structures is CO-Layer structure(-23.64 kJ/mol) > OH-Layer structure(-19.44 kJ/mol) > COOH-Layer structure(-18.28 kJ/mol)，the above values are all lower than that of CH4 in non-functionalized C-layer structure(-25.37 kJ/mol). The presence of oxygen-containing functional groups weakens the CH4 adsorption performance of coal, and the strength of the effect is related to the basicity and hydrophobicity of the functional groups. The order of adsorption energy of CO2 in different oxygen-containing functionalized structures is COOH-Layer structure(-36.33 kJ/mol) > OH-Layer structure(-34.06 kJ/mol) > O-Layer structure(-33.43 kJ/mol), the above values are higher than that of CO2 in non-functional C-layer structure(-32.54 kJ/mol); the existence of oxygen-containing functional groups improve the CO2 adsorption performance of coal, and the strength of the influence is related to the polarity of the functional groups. The adsorption energy of CH4 in the functionalized pyridine nitrogen structure is -31.00 kJ/mol, which is higher than that in the non-functionalized C-layer structure. The adsorption energy of CH4 in the functionalized pyrrole nitrogen structure is -6.40 kJ·mol, which is less than that in the non-functionalized C-layer structure. The adsorption energies of CO2 in pyridine nitrogen functionalized structure and pyrrole nitrogen functionalized structure are -50.56 kJ/mol and -14.71 kJ/mol, respectively. The hindrance and promotion of nitrogen-containing functional groups on the adsorption of CH4 and CO2 in coal structure have the same law, that is, pyridine promotes adsorption and pyrrole hinders adsorption.